This invention relates to a device which converts an input frequency to a frequency which has been offset by a given amount determined by an offset frequency. The output of the device is a pulse train representative of a single sideband transmission at the offset frequency. This device is particularly useful where it is desired to receive a return signal from a transmitted signal where the transmitted signal is expected to shift in a random manner across a given frequency range. By offsetting the frequency of the returned signal in accordance with the change in frequency of the transmitted signal, a converted frequency can be obtained which is maintained within a narrow band width so that accurate measurements may be made of the return signal.
In the prior art, sideband signals have generally been produced by a filter method and by a phasing method. The filter method involves the use of a band pass filter having sufficient selectivity to pass one sideband and reject the other. Normally, (modulated) oscillator output at a filter frequency is filtered so that only the upper or lower sideband appears at the output. This resulting signal is mixed with an RF oscillator to produce a desired output frequency.
According to the phasing method, a modulating signal is split into two components having a 90 degree phase difference. An oscillator output is also split into two phases having a 90 degree phase separation. The modulation signal phases are combined with the oscillator signal phases in balanced modulators and the combined output results in one sideband being cancelled and the other being augmented. In each of these arrangements, fine adjustments must be made either to the filter or to the phasing and balanced modulators.
In tracking signals having varying frequencies it is necessary to use one of various methods to receive the varying frequencies with a single tuner. The most common technique is to heterodyne the signal, thereby allowing a received signal to be detected with a high degree of selectivity, even though the receiver must be able to detect signals at more than one frequency. In the case of random noise-generated transmissions, rapid responses are needed to track the signal. If such a signal can be accurately tracked, despite large variations in its frequency, an anti-jamming capability is achieved. The use of a single sideband also increases the signal-to-noise ratio of a detected signal, provided that the single sideband can be accurately detected by detecting circuitry.
Normally, shifting in the frequency of a single sideband transmission results in difficulties in properly tuning the transmission. For this reason, it is desired that an accurate and reliable means be provided to generate a single sideband signal and to overcome the problems caused by a signal varying in frequency as it is being received.